These are laid out very well in Tim Wu's 2020 White House Executive Order on competition, which identifies 72 ways the agencies can act against monopoly without having to wait for Congress:
But of course, the majority of antimonopoly power is vested in the FTC, the agency created to police corporate power. Section 5 of the FTC Act grants the agency the power to act to prevent "unfair and deceptive methods of competition":